Means for indicating faults in the crucible of induction melting furnaces



May 25, 1955 BECKIUS 3,185,394

MEANS FOR INDICATING FAULTS IN THE CRUCIBLE 0F INDUCTION MELTINGFURNACES Filed Aug. 10, 1961 IN VEN TOR. Iv r 35 c )CI I Maw/g UnitedStates Patent 6 Claims: (Cl. 317-9 Hitherto known systems for thesupervision of the condition of the crucible for induction meltingfurnaces are based upon the measuring of the leakage current passingfrom the furnace coil and through the melt to ground. In order to bringabout such a current, either a separate voltage source is used, in whichcase the leakage current is relatively independent of which part of thecrucible has undergone deterioration in its condition, or theoperational voltage of the furnace is used. In both cases a connectionlead, which is always required for continuous supervision, has to betraced between the furnace circuit and ground. The leakage current,however, in the latter case will be dependent on the potential of thatpart of the furnace coil in the vicinity of which the deterioration ofthe crucible has taken place. The first mentioned system therefore isdeemed more advantageous, and the present invention relates to such asystem.

It is usual to indicate the condition of the crucible by directmeasuring of the leakage current. If a fault on the crucible has becomeso consider-able that direct short-circuiting occurs between the meltand the furnace coil, or if the resistance of the crucible is decreasedconsiderably, the risk occurs at continuous supervision that theindicating means becomes overloaded or destroyed, since it isdimensioned for the measuring of very feeble currents.

The grounding of the furnace circuit implies that the melt, due to afault of the crucible, is under tension and gets a potential againstground, for although the grounding is brought about by the melt it isnot completely certain whether this ground connection exists or not. Thefurnace operator handling the furnace and who, by means of rods or thelike, may come into contact with the coil, the melt or the crucible wallmay therefore be exposed to danger of coming into contact with thetension. Particular care must be taken if comparatively high voltagesand low frequencies in the furnace circuit are concerned. Therefore onehas, as a rule, not employed crucible fault indication means forcontinuous supervision, but carried out the furnace circuits withoutgrounding in order to minimize the danger of contact with the melt, orground circuits have been intermittently coupled in for short con tactsof the coil insulation or the like.

The present invention relates to a crucible fault indicating means fedwith a separate auxiliary voltage such as from an alternating currentnetwork, into an indicating circuit. The means is based on the fact thatthe increased leakage current which occurs at the occurrence of a faultof the crucible, bring about a voltage drop in a resistance seriesconnected in the auxiliary voltage circuit. The auxiliary circuitcomprises one Winding of an insulating transformer, the other winding ofwhich is inter-connected between ground and a point of the furnacecircuit.

The feature of the invention resides in that between said winding andsaid point there is inter-connected an electric blocking circuit whichis tuned to the frequency of the furnace circuit and is so dimensionedthat the current passing through the blocking circuit is maintainedbelow a value dangerous for a human being, usually below 10 ma.

The condition of the crucible is indicated by measuring 3,185,894Patented May 25, 1965 the said voltage drop across the said resistance.This voltage drop may also be used for interrupting the furnace circuitif required. When there is no insulating fault between the coil and themelt no current flows in the auxiliary voltage circuit and the voltagerelations in said circuit are suitable, for example a certain voltage ispresent across the primary winding. When an insulation fault between thecoil and the melt occurs, a current flows in the auxiliary circuit and acertain voltage drop takes place over said series resistance. Thevoltage relations in the circuit are thus changed and, for example, thevoltage across the primary winding is changed. Said changed voltage maybe indicated in different ways, for example, by means of relays utilizedfor switching-off the power circuit. The indicating device of the faultindicating circuit, which may be a voltage meter, is not overloaded evenin the event that a stronger current occurs, for example, at directshort-circuiting between the melt and the furnace coil.

The blocking circuit is tuned to the frequency of the power circuit andso dimensioned that, at such frequencies and frequencies close to thisfrequency the current passing through the blocking circuit is maintainedbelow the value which is dangerous when passing through a human body. Acurrent not stronger than 10 me. pass ing through the human body when incontact with an electric mains may be considered as not dangerous. Theblocking circuit has not only the purpose of eliminating the danger forthe personnel when the furnace is under operation but also of preventingcapacitively transmitted voltage of furnace frequency disturbing theindication of i the condition of the crucible.

There must be a certain difference between the frequency of the furnacecircuit and the frequency of the auxiliary voltage in order to give themeans the required sensitivity. If the ratio between the frequencies ofthe furnace circuit and of the auxiliary voltage circuit is less than40:1, a rectifier is used in series with the blocking circuit fortransforming the alternating auxiliary indicating current into directcurrent before passing through the blocking circuit and the resistanceof the crucible. The auxiliary current circuit is thereby influenced toa minor degree by the impedance of the blocking circuit.

In furnace constructions where the conductors of the furnace coil arecompletely surrounded or are covered on the side directed against themelt by an electrically conductive shield which i not in electricallyconducting contact with either the furnace coil or the melt, the shieldis connected to one pole of the blocking circuit whereby capacitivetransmission of disturbances from the furnace to the indicating circuitcan be eliminated.

The means according to the invention will be described with reference tothe accompanying drawings in which FIGURES 1, 2, 3 and 4 illustrate theabove examples of carrying out the invention.

A coreless furnace with a crucible 3, surrounded by a coil 2 is fed froman AC. source, which, in FIG. 1, is of high frequency. In such furnacesthere is a need to detect insulation failures between the coil 2 and themelt. Therefore at the crucible 3 a ground connection 8 is arranged.Circuit 1 in FIGURE 1 is consequently a power circuit. From one of thepower lines a connection to the ground connection through the secondarywinding of an insulation transformer 7 is arranged. The primary windingof this transformer is connected to a low frequency auxiliary network(at 6) in series with a resistance 18. The voltage over the primarywinding of the transformer 7 is measured at It) and indicated inparallel at 9 by means of a relay, a signal device, or the like. Thesecondary circuit of transformer 7 is connected over a very highresistance to the melt when the coil is satisfactorily insulated. Inthis case almost no current is fed to the i11- dioating circuit overtransformer 7 and thus the voltage drop over resistance 18 isnegligible, as there is no current in the primary winding of theauxiliary circuit. If the insulation breaks down, the resistance betweenthe coil and the melt decreases and a current from the transformer flowsin the indication circuit secondary winding of transformer 7powerline-coil 2melt-ground S secondary winding at 7. At the same time acurrent flows in the primary winding of transformer 7 through resistance18 and thus a voltage drop takes place across resistance 18 and thechanged voltage relations are measured at 10 and 9. The relay at 9 isthus actuated and the power in circuit 1 is switched off (not shown).The circuit 7123-8-7 is called a grounded error protective circuit orindication circuit and protects the furnace from the consequences ofinsulation faults between the coil and the melt.

There is, however, a drawback connected with such a protective circuitwith ground connection. Suppose a person by means of a metallic rod orthe like contacts the melt or coil when an insulation fault occurs andconductor S has been broken. In this case a current from the A.C. sourceat 1 runs through such person to ground and from ground through thesecondary winding at 7 to the power line at 1, and said current may bedangerous. In order to protect said person from the risk of a brokenground connection (at 8), a secondary protective means or blockingcircuit for limiting current from the source in circuit 1 and at thefrequency of said source is inserted in aid protective circuit. Theblocking circuit comprises a condenser 11, a reactance coil 12, and anindicating device 13 for indicating the state of the tuning of theblocking circuit. Said secondary protective means or blocking circuit istuned in relation to the frequency in the circuit 1 in such a mannerthat no current through said blocking circuit can exceed dangerousvalues, such as ma. Current from circuit 4 over transformer 7 is notblocked through said blocking device but such current of appreciabledegree is only present where insulation faults between the coil and themelt occur and then only for a short moment before the power in circuit1 is switched off.

According to the example shown in FIGURE 2 it is assumed that thefurnace circuit is supplied with commercial network frequency. Theauxiliary voltage supplied to the indicating circuit and taken from thenetwork is rectified by a rectifier means 14 inter-connected on thesecondary winding of the insulating transformer 7, and provided with aspark gap device 15 which indicate any diminished voltage drops over therectifier bridge which may possibly occur during operation. In FIG. 2the frequency of the power circuit 1 is network frequency (60 or 50cycles) and in auxiliary circuit 4 network fre- I quency, rectifiedthrough bridge 14. In this case the blocking means is tuned for blockingnetwork frequency, but DC. from bridge 14 is not blocked. FIGURE 3 showsan embodiment wherein a voltage divider 16 is inter-connected betweenthe furnace circuit and the blocking circuit. The said point of thevoltage divider is connected to the blocking circuit while the terminalsare connected to the supply leads of the furnace coil 2. By thisarrangement more favourable dimensioning conditions for the blockingcircuit are obtained.

According to FIGURE 4, an electrically conducting shield 17 isinterposed between the furnace coil and the crucible. The blockingcircuit is connected to said shield, since this controls the insulationand homogenity of the crucible wall, and there must be a largeresistance be- The embodiments described may be used in combi A nation.

Although the means according to the invention has been described for usein an induction furnace for melting purposes it may also be employed incombination with other induction furnaces such as furnaces for warm ingpurposes, wherein the charge is surrounded by a coil with insulated ornot insulated conductors and where contact between the conductors andthe charge is to be indicated.

I claim:

1. In combination with a crucible having an induction coil adjacentthereto and an A.C. power supply connected to the coil for feedingcurrent thereto, a connection from the crucible to ground, an indicatorcircuit for indicating an insulation fault in the crucible comprising atransformer having a secondary winding connected between said groundconnection and said power supply, an auxili-ary A.C. source, meansincluding a resistance connecting said auxiliary source to the primarywinding of said transformer, measuring means in said connecting means,said measuring means being responsive to the flow of current occurringin said connecting means upon a break-down in insulation between thecoil and the crucible, and blocking means coupled in series with thesecondary winding of the transformer, said blocking means being tuned tothe frequency of the power supply and including means to keep the flowof current therethrough at such frequency relatively low withoutsubstantially impeding flow of current in said connecting means.

2. A device according to claim 1, in which the blocking means comprisesa capacitor in parallel with a reactor, said blocking means beingcoupled in series between one terminal of the secondary winding of saidtransformer and said power supply.

3. In a device according to claim 1, rectifier means between thesecondary of the transformer and the power supply and ground forrectifying current fed from said auxiliary A.C. source into saidtransformer.

4. Means according toclaim 1, wherein the ratio between the frequenciesof the power supply circuit and of the connecting means is small, andthe blocking means includes a rectifier connected to the secondarytransformer.

5. Means according to claim 1, in which the blocking means is connectedto the power supply by a voltage divider, the mid point of which isconnected to the blocking means, while its terminals in parallel areconnected to the power supply.

6. Means according to claim 1, in which the furnace coil is providedwith an electrically conducting shield insulated from the conductors ofthe furnace coil and from the melt and connected to the blocking means.

References Cited by the Examiner UNITED STATES PATENTS 1,778,398 10/30Northrup 317-9 X 1,801,790 4/ 31 Breisky .et al. 340242 X 2,474,711 6/49Yonkers et al. 317-9 X 2,614,200 10/52 McNair 317-9 X 2,990,542 6/61Seitz -Q. 1327 X SAMUEL BERNSTEIN, Primary Examiner. LLOYD McCOLLUM,Examiner.

1. IN COMBINATION WITH A CRUCIBLE HAVING AN INDUCTION COIL ADJACENTTHERETO AND AN A.C. POWER SUPPLY CONNECTED TO THE COIL FOR FEEDINGCURRENT THERETO, A CONNECTION FROM THE CRUCIBLE TO GROUND, AN INDICATORCIRCUIT FOR INDICATING AN INSULATION FAULT IN THE CRUCIBLE COMPRISING ATRANSFORMER HAVING A SECONDARY WINDING CONNECTED BETWEEN SAID GROUNDCONNECTION AND SAID POWER SUPPLY, AN AUXILIARY A.C. SOURCE, MEANSINCLUDING A RESISTANCE CONNECTING SAID AUXILIARY SOURCE TO THE PRIMARYWINDING OF SAID TRANSFORMER, MEASURING MEANS IN SAID CONNECTING MEANS,SAID MEASURING MEANS BEING RESPONSIVE TO THE FLOW OF CURRENT